Computer tomography (CT, also called computed tomography) has evolved into a commonly used means, when it comes to generating a three-dimensional image of the internals of an object. The three-dimensional image is created based on a large number of two-dimensional X-ray images taken around a single axis of rotation. While CT is most commonly used for medical diagnosis of the human body, it has also been found applicable for non-destructive materials testing. Detailed information regarding the basics and the application of CT, can be found in the book “Computed Tomography” by Willi A. Kalender, ISBN 3-89578-216-5.
One of the key innovative aspects in future CT and XR imaging is the energy-resolved counting of the photons which are let through or transmitted by the object being analyzed when being exposed to X-ray radiation. Depending on the number and energy the transmitted photons have, it can be concluded through which type of material the X-ray radiation has traveled. In particular, this allows to identify different parts, tissues and materials within a human body. When the detection or counting of photons is referenced, it is understood, that when a photon impinges on the conversion material of a sensor, it creates a charge pulse (sometimes also referred to as current pulse). This charge pulse is detected and the presence of a photon is concluded. The charge pulse results from a larger number of electron-hole pairs, which are generated, when an X-ray photon interacts with the sensor conversion material. The duration of this charge pulse corresponds to the so-called charge collection time. Detection of single electron-hole pairs is not in the focus of this application, but the processing of a charge pulse resulting from electron-hole pairs representing a photon, which may also be expressed by the formulations “detecting photons” or “counting photons”. For a charge pulse, which is generated by interaction of an X-ray photon, also the formulation is used that the charge pulse belongs to this X-ray photon. Along the same lines, e.g. “processing a charge pulse caused by a photon impinging on the sensor” is sometimes also denoted as “processing a photon” in the following.
In order to count single X-ray photons with determining the photon energy, at least one sensor pixel is used which is connected to a counting detector. The counting detector is an electronics circuitry that provides the following functionalities:                a) a preamplifier amplifies and/or integrates the charge pulse resulting from a displacement/movement of charged particles (electrons and holes) in a conversion material, e.g. CZT (cadmium zinc telluride) caused by the incoming photons,        b) a shaper (shaping element) which forms a voltage pulse from the output of the preamplifier, the height of the voltage pulse being proportional to the amount of charge carried by the charge pulse,        c) a number of discriminators, which check whether the voltage pulse at the output of the shaper is above or below the thresholds defined by each discriminator, and        d) for each discriminator a counter, which counts the pulses, that have exceeded the threshold defined by the discriminator.        
One of the fundamental challenges of counting X-ray photons e.g. in a computer tomograph is the very high rate of incoming X-ray photons (e.g. in the order of 109 quanta/s·mm2 for the direct beam hitting the detector and considering tube filtration). This means, that within a very small time window an extremely large number of photons has to be discriminated based on their corresponding charge pulses in order to be able to count the photons.
One approach of coping with the large number of photons is to reduce the size of the sensor pixel thereby reducing the number of photons received per pixel. However, given that currently a practical pixel edge length is in the order of 200 μm to 400 μm, the number of photons per pixel still remains too high to allow for a correct counting.
Another issue that has to be dealt with is related to leakage (or dark) currents that are present when a direct converting material is run at a high reverse bias voltage. The leakage current must be stopped from entering the preamplifier, since otherwise the preamplifier will become overloaded and can no longer form voltage pulses.
For the special case of micro-strip counting detectors an AC (alternating current) coupling capacitor arranged between the sensor and the shaper, blocks the leakage current from entering the preamplifier input. However, in order to represent a very low impedance for the time-varying “AC” current, which results from the sequence of charge pulses generated by X-ray photon interaction with the sensor material, the capacitor must be big and therefore is not convenient or not practical for detectors having a pixel size as mentioned before.
Instead, attempts have been made to include some circuitry for leakage current compensation (LCC), which can be built within the pixel to drain the sensor's leakage current.
Coming back to the counting detector, a prominent issue is the so-called paralysis of the counting detector. This term describes the situation, when a second photon is coming in while the preamplifier is still processing a first photon. This means, that the occurrence of the second photon falls within the processing time (which is determined by the shaping time) of the first photon. This results in a voltage pulse being formed, which is considerably longer in time than what the voltage pulse would have looked like, if the first photon had been processed alone.
In addition, further charge pulses, which come in during the ongoing processing will further increase the duration of the voltage pulse at the output of the preamplifier. Consequently, the counting detector sees a combined voltage pulse, with an increased duration compared to the duration if only the first photon had been processed. The voltage pulse may even become so broad, that, although the very first and the very last photon in a sequence of photons are more than the processing time for a photon apart from each other, these photons cannot be distinguished. A further disadvantage of the pulse pile-up described above is that the combined voltage pulse of subsequent photon events is higher than any of the single pulses. If this effect enables the pulse to exceed a discriminator threshold, a false event is registered.
As a result, only one photon can be counted, even though more photons have actually interacted with the sensor material. Hence, the counting detector is paralyzed by subsequent charge pulses which arrive shortly after each other during the processing time of a single photon. The time, during which the detector is not able to discriminate further photons, since it is occupied by processing a photon, is also called the dead-time of the counting detector. The dead-time behavior described above is often also called paralyzable or extendable dead-time.
It should be noted, that it is possible to build preamplifiers and shapers which can process signals at a higher speed. However, such would compromise the noise figures and worsen the ballistic deficit leading to a deterioration of the energy information. For details on the term “ballistic deficit”, cf. G. Knoll, “Radiation Detection and Measurement”, 3rd edition, J. Wiley & Sons, 1999.
One approach on how to address this problem is disclosed in “First Experimental Characterization of ROTOR: the New Switched-Current VLSI Amplifier for X-ray Spectroscopy with Silicon Drift Detectors” by C. Fiorini, A. Pullia, E. Gatti, A. Longoni and W. Buttler, IEEE Transactions on Nuclear Science, Vol. 47, No. 3, pages 823-828, June 2000. It is proposed to filter the sensor signal by using a certain number of processors working in parallel, each one performing a trapezoidal weight function. The weight functions are suitably shifted one with respect to the other in order that for any arrival time of the event, there is always at least one processor that is amplifying the signal with a maximum gain, i.e. the signal occurs in correspondence of the flat-top of its trapezoidal weight function. The outputs of the four processors, called wheels, are sequentially sampled in a common holding capacitor.